1. Field of the Invention
The invention relates generally to a piezoelectric element such as a piezoelectric resonator or a SAW (Surface Acoustic Wave) device such as a SAW resonator and a SAW filter constructed of a piezoelectric crystal such as quartz crystal or a piezoelectric ceramic material. More particularly, the invention relates to a piezoelectric element manufactured by utilizing an excited active species generated in plasma at approximately atmospheric pressure to perform surface treatment, and a method of manufacturing the same.
2. Description of the Related Art
A resonance type piezoelectric element which operates based upon the piezoelectric effect is conventionally well known in the art and may be used to function as a resonator, a filter or an oscillator. Piezoelectric elements include tuning fork type resonators using a frequency band ranging from about 20 kHz to 1 MHz, piezoelectric resonators such as AT-cut resonators using a frequency band within a range of from about 4 to 125 MHz, and SAW devices using a surface acoustic wave. In general, a piezoelectric resonator unit comprises a resonator, which in turn comprises a piezoelectric piece having an electrode formed on the surface thereof, and a plug having a lead for electrically connecting the electrode to an external electric circuit. The resonator is sealed in an air-tight manner in a case by a plug, which is a hermetic terminal.
A SAW device, which permits stable achievement of a high frequency within a range of from 100 MHz to the GHz band, may be used in a SAW resonator when constructing an oscillating circuit for a SAW filter to operate at high frequencies. In general, a SAW device has a SAW piece which further comprises a piezoelectric substrate such as a quartz plate. The quartz plate is further formed with a fine comb-shaped electrode on the surface thereof. The SAW piece is then usually bonded along its entire surface onto a support. This mechanism is then sealed in an air-tight manner in a case. Recently, the present inventors have developed a SAW device having a construction in which a SAW piece is cantilever-mounted on a plug comprising a hermetic terminal. This design is similar to that of the piezoelectric resonator unit. Then this improved SAW device is vacuum-sealed in a case.
During the construction of these piezoelectric elements, foreign matter may be mixed into the case before the piezoelectric resonator or SAW piece is sealed in the case in an air-tight manner. Also, it is possible that the surface coating of the plug or the case may peel off, producing dust. In the case of a tuning fork type piezoelectric resonator or a SAW device, dust may adhere to the piezoelectric piece surface, tending to cause a short circuit between adjacent electrodes. Thus, there is a risk that as a result, the oscillation of the resonator may stop, or the electrical impedance of the resonator may change. Particularly in view of the recent requirement for the downsizing and miniaturization of electronic parts, the gap between electrodes is becoming smaller, shrinking to about 20 to 30 .mu.m, for example, to achieve a more compact piezoelectric element. Thus the tendency of short circuit between electrodes is increased.
Additionally, the electrodes may deteriorate under the effect of oxygen entering the case when the resonator is sealed within the case, or as a result of the adsorption of gases released from the plug or the case after being sealed. This deterioration may result in a shift of the oscillation frequency, or an increase in the CI (crystal impedance)-value. This may in turn cause deterioration of the operating properties of the piezoelectric element and may reduce the reliability of the piezoelectric element.
Piezoelectric elements comprising a piezoelectric piece with an electrode formed thereon and an insulating film formed on the surface of the piezoelectric piece have been developed in the prior art. For example, Japanese Examined Patent Publication No. 2-22564 discloses a SAW element in which a degradation in the operating properties which may be caused by short-circuiting of the electrodes is inhibited by coating the electrode portion formed on the piezoelectric substrate with a tantalum pentoxide insulating film, which is formed by sputtering vapor deposition. Japanese Patent Application No. 3-291660 laid open on May 25, 1993, which has been filed by the present inventors, discloses a construction in which a change in oscillation frequency or a deterioration in the operating properties, such as the CI-value, caused by adsorption of external gases or dust, is prevented by forming an insulating film comprising an oxide such as SiO.sub.2 or fluoride on the surface of at least a flat electrode of a tuning fork type piezoelectric resonator. This film is formed by vacuum deposition, sputtering, or vacuum-plasma CVD. Japanese Unexamined Patent Publication No. 60-134617 discloses a piezoelectric resonator wherein deterioration of the resonator caused by oxidation or the like is prevented by applying a polyimide onto a metal layer, which has in turn been vapor-deposited on the surface of the piezoelectric resonator. Thus, an excitation electrode and an outgoing electrode are formed by patterning using the photolithography technique. This in turn causes the polyimide resin film to remain on these electrodes. A method of manufacturing this prior art piezoelectric resonator is also provided.
In conventional piezoelectric elements, when an insulating film formed on an electrode of the piezoelectric element is formed of a hard and rigid oxide such as SiO.sub.2, the excessive thickness of the insulating film may hinder vibration of the piezoelectric element piece, thus reducing the CI-value and resulting in a decreased efficiency of operation. Thus, a reduced thickness insulating film having a thickness of about 1,000 .ANG. is utilized to improve the efficiency of operation. However, use of this reduced thickness insulating film may result in insufficient electric insulation of the piezoelectric element. Furthermore, because of the high accuracy requirement of about .+-.200 .ANG. in the film thickness, and the need to eliminate irregularities in the thickness of the film, it is difficult to control the film thickness to the required degree during the manufacturing process.
Particularly, in the case of a tuning fork type piezoelectric resonator unit, it is the conventional practice to adjust the frequency of the unit by removing the metal film overlayed on the electrode by the use of a laser beam. However, the laser beam cannot remove a conventional insulating film comprising an oxide such as SiO.sub.2 since the laser is transmitted though this oxide film. Therefore, the conventional frequency adjusting technique of using a laser beam does not work when an oxide such as SiO.sub.2 is used as the insulating film material. Therefore, it is necessary to form and remove portions of the insulating film by means other than by a laser beam. For example, a mask or the like may be used, thus resulting in a more complicated manufacturing operation.
When forming an insulating film on an electrode before mounting the piezoelectric element piece and electrode on the plug, the land for electrically connecting the piezoelectric electrode to the plug must be masked, leading to a complicated film forming operation. After being sealed in the case, metal portions such as the land and an inner lead terminal of the electrode which are exposed to the interior of the case without being covered with the insulating film may produce metal scrap, which may in turn cause a short circuit of the adjacent electrodes.
On the other hand, if the insulating film is formed on the piezoelectric element piece after mounting the element piece on the plug, the formed insulating film may cover a portion of the sealing portion of the plug. Thus, it would be impossible to seal the case in an air-tight manner. While it is therefore necessary to mask the plug upon forming the insulating film and to form the insulating film before mounting the element to the plug, it is difficult to conduct a complete masking during manufacture. Such a masking, if applied, would be of a large scale, resulting in a decreased efficiency of production.
A film formed by sputtering or vapor deposition requires expensive vacuum equipment during operation, hence requiring much time and cost for manufacturing. It is also difficult to conduct continuous processing of piezoelectric elements with these vacuum-type manufacturing processes of the piezoelectric element. It is thus impossible to automate the manufacturing process or to improve the productivity of manufacture. Furthermore, because the batch processing of a plurality of piezoelectric elements is usually effected under a vacuum or at a reduced-pressure condition, the operating frequency of the element piece cannot be measured while the insulating film is being formed. This leads to a serious difficulty in controlling the film thickness. Particularly when utilizing sputtering to form the film, the degree of film adhesion varies with the surface condition of the piezoelectric resonator and electrode to be formed. This may affect the operating frequency property of the piezoelectric element upon aging of the element. It is therefore necessary to carry out a sufficient pretreatment to insure proper film adhesion. However, it is still impossible to measure the operating frequency of the element piece while the insulating film is being formed.
Additionally, for the piezoelectric resonator disclosed in Japanese Unexamined Patent Publication No. 60-134617, formation of the insulating film and measurement of frequency cannot be accomplished simultaneously, since the manufacturing process is conducted in a batch manner. Another inconvenience is that the resin film is formed on the piezoelectric element before the element is mounted on the plug. It is therefore necessary to remove the resin film from the outgoing electrode surface upon connection of the element to the plug. As the connecting portion of the plug and plug surface cannot be coated prior to being connected, these portions exposed to the interior of the case may deteriorate by oxidation or may produce dust, which may in turn cause a short circuit between the outgoing electrodes. Therefore, it would be beneficial to provide a piezoelectric resonator, and a method of forming this resonator, which overcomes these drawbacks of the prior art.